In the prior art, use of a number of catalysts in the oxidative coupling of methane to C.sub.2 -hydrocarbons (i.e. ethane and ethylene) and in the oxidative dehydrogenation of C.sub.2 -C.sub.4 alkanes is known.
At present, ethylene, which is a keystone to the petrochemicals, from light paraffins (C.sub.2 -C.sub.4 paraffins) is produced commercially by a well established process based on the thermal cracking of ethane or C.sub.2 -C.sub.4 paraffins at 750.degree.-1000.degree. C. The cracking process is highly endothermic and involves coke deposition (L. Kniel, O. Winter and K. Stork in Chemical Industries, Vol. 2 "Ethylene: Key-Stone to the Petrochemical Industry, Marcel Dekkor, Inc., New York and Basel). Typical results given in the above reference indicate that at 50% and 60% conversion of ethane with 83.3% and 80.0% selectivity for ethylene, respectively, could be obtained by the thermal cracking of ethane.
The process for production of ethylene, based on thermal cracking of light paraffins have following limitations: (1) They are highly endothermic and hence are energy intensive, (2) They involve extensive coke deposition inside the pyrolysis reactor tubes, thus causing increase in pressure drop and frequent break downs.
Catalytic processes based on oxidative dehydrogenation of ethane or light paraffins are also known in the prior art. A number of catalysts containing low melting compounds such as alkali metal compounds, metal halides and other low melting metal oxides are known for the oxidative dehydrogenation of ethane or lower alkanes (Ref. Eastman and Kolts, U.S. Pat. No. 4,310,717 (1982); Eastman, U.S. Pat. No. 4,368,346 (1983); Eastman and Kimble, U.S. Pat. No. 4,450,313 (1985); Kimble, U.S. Pat. No. 4,476,344 (1984); Eastman et al., U.S. Pat. No. 4,497,971 (1985); Jpn. Kokai Tokyo Koho JP 61,282,322 (1986); Kolts and Guillory, Eur. Pat. Appl EP 205,765 (1986). Because of the presence of the low melting or volatile component, these catalysts are deactivated during the process due to the loss of active components from the catalyst by evaporation at hot spots and/or due to the catalyst sintering.
Methane is a major constituent of natural gas and also of bio-gas. Processed natural gas typically comprises a major amount of methane and minor amounts of ethane, propane, butanes, CO.sub.2 and nitrogen and very small amount of pentanes. Methane or natural gas is used principally as a source of heat in commercial, industrial and residential services and also a source of hydrogen for the fertilizer industries and syngas (CO and H.sub.2) for the production of methanol and in the Fischer-Tropcsh synthesis. World reserves of natural gas are constantly being upgraded and more and more natural gas is being discovered than oil. Because of the problem associated with transportation of a very large volume of natural gas, most of the natural gas produced particularly at remote places is flared. This causes not only a wastage of valuable energy but also causes a global warming due to release of large amount of CO.sub.2 in the atmosphere. If energy efficient commercially feasible processes were available for converting methane or natural gas into value added products like ethylene and other lower olefins, which can be converted to easily transportable petrochemicals and liquid hydrocarbons by known processes, this can have far reaching economic impact and also the exploration of more gas-rich fields could greatly increase the natural gas reserves.
Researchers at Atlantic Richfield Co. (USA) patented a series of supported reducible metal oxides, such as oxides of Mn, Bi, Ge, In, Pb, Sb, Sn and Pr with or without alkali metals supported on SiO.sub.2 or MgO, as catalysts for the oxidative conversion of methane to ethane and ethylene, utilising a so-called "Redox" approach, which use lattice oxygen from the catalyst to perform the oxidative coupling of methane to C.sub.2 -hydrocarbons at about 800.degree. C. and gas hourly space velocity of about 860 h.sup.-1 (Ref. Jones and Co-workers, J. Catal., 103, 302-319 (1987); Jones et.al., Energy and fuels, 1, 12-16 (1987); Jones et.al., U.S. Pat. Nos. 4,443,644; 4,443,645; 4,443,646; 4,443,647; 4,443,648; 4,443,649; 4,443,649; 4,499,322; 4,444,984 (1984); 4,495,347; 4,517,398 (1985); Withers, et. al, U.S. Pat. No. 4,634,800 (1987); Sofranko and Jones, U.S. Pat. No. 4,544,784 (1987). These catalysts are based on reducible metal oxides and are used in a stoichiometric fashion by atlernatively exposing them to an oxidising atmosphere and then to methane in the absence of free oxygen. The main drawbacks of the processes using reducible metal oxides as catalysts following "Redox" approach in the oxidative conversion of methane are as follows:
(i) The "Redox" operation is very complicated and requires a complicated reactor consisting of physically separate zones for a methane contacting step and for an oxygen contacting step, with an arrangement for recirculating the catalyst between two zones.
(ii) Because of the "Redeox" operation and requirement of lower space velocity of methane to achieve reasonable methane conversion, the productivity of ethane and ethylene is very low.
(iii) The "Redox" catalyst without containing alkali metals showed very poor activity and selectivity in the oxidative methane coupling process; whereas the alkali metal containing catalysts showed good activity and selectivity but the alkali metal containing catalysts are expected to be deactivated fast because of the evaporation of alkali metals and also due to the catalyst sintering during the high temperature process operation.
A number of alkali metal promoted catalysts, such as alkali metal containing alkaline earth metal oxides, group III and IV metal oxides and the catalysts containing low melting metal compounds have been described for the oxidative conversion of methane to C.sub.2 -hydrocarbons in presence of free oxygen (Ref. Kimble and Kolts, Chemtech, 501 (1987); Baerns et. al., U.S. Pat. No. 4,608,449 (1986); Leyshon et.al., U.S. Pat. No. 4,801,762 (1989); Devries et. al., U.S. Pat. No. 4,814,538 (1989); Kimble et. al., U.S. Pat. No. 5,087,787 (1992); Lunsford and Hinson, U.S. Pat. No. 5,041,405 (1991). The main drawback of the use of these catalysts in the oxidative conversion of methane to higher hydrocarbons are that the catalysts are deactivated during the process because of the loss of active components from the catalyst by evaporation and also due to sintering of the catalysts during their operation in the high temperature process.
Recently in U.S. Pat. No. 4,822,944 (1989) Brazdil Jr., et.al. have disclosed a novel oxidative methane coupling catalyst, having melting points above 900.degree. C., represented by the formula: SrLa.sub.n O.sub.x, wherein n is a number in the range of about 0.1 to about 100, and x is the number of oxygens needed to fulfil the valence requirements of the other elements. The catalyst is prepared generally by mixing an aqueous solution of compounds containing the metal components, forming a precipitate, drying this precipitate and calcining to produce desirable physical properties. Although, the catalyst disclosed by Brazdil Jr. et.al. has high thermal stability against evaporation of active catalyst component during the oxidative methane coupling process occurring at high temperature (at about 900.degree. C.), the catalyst shows poor activity and also poor selectivity in the oxidative coupling of methane; for example, at the catalyst temperature of 915.degree.-916.degree. C. and the gas hourly space velocity of 23,570 cm.sup.3.g.sup.-1.h.sup.-1, the methane conversion, C.sub.2 + yield and C.sub.2 + selectivity are 14.2-14.6%, 8.7-9.1% and 61.2-62.8% respectively. Further the catalyst operates in the oxidative methane coupling process at high temperature about 900.degree. C. and yet shows very poor catalytic activity. Hence, there is a need to develop an improved catalyst which is not only thermally stable at high temperature but also highly active and selective in the oxidative coupling of methane to higher hydrocarbons.
The price of methane or natural gas is high and it is increasing day-by-day because of energy crisis. Hence, the oxidative conversion of methane or natural gas to ethylene will be economically feasible only if the selectivity and productivity for ethylene in the conversion process are high and when process is operated continuously for a long period without catalyst deactivation and/or fouling due to a loss of catalytic activity or mechanical strength. In the oxidative conversion of methane to ethylene, two methane molecules are coupled to form ethane which is then dehydrogenated to ethylene. Therefore, in order to make catalytic oxidative methane-or natural gas-to-ethylene conversion process economically feasible and/or competitive to the conventional processes used for the production of ethylene, there is a need to use an improved catalyst, preferably an improved supported catalyst, which is thermally and hydrothermally stable at the process operating conditions, has high mechanical strength and has high activity and productivity with very high selectivity not only in the oxidative coupling of methane to ethane but also in the oxidative dehydrogenation of ethane to ethylene, and also has long life in the process.
The present invention is directed to overcome the limitations and drawbacks of the earlier catalysts thereby providing a novel improved supported catalyst.
The objective of the invention is to provide an improved supported catalyst comprising strontium and rare earth oxides deposited on a sintered low surface area porous catalyst carrier (or support).
Another objective of the invention is to provide an improved supported catalyst having higher activity, selectivity and efficiency (or productivity) in the oxidative coupling of methane to higher hydrocarbons (C.sub.2 +-hydrocarbons) in the presence of free oxygen without catalyst deactivation or fouling during the process.
The other objective of the invention is to provide an improved supported catalyst having high activity, selectivity and productivity in the oxidative conversion of natural gas to ethylene and other lower olefin (i.e. propylene and butylenes) and in the oxidative dehydrogenation of ethane and other lower alkanes (i.e. propane and butanes) to the corresponding olefin and also has high thermal and hydrothermal stability during the operation of these oxidative methane, natural gas and lower alkanes conversion processes.
Another objective of the present invention is to provide a process for the manufacture of an improved supported catalyst.
Another objective of the present invention is to provide an improved supported catalyst which operates at a temperature below 900.degree. C. with higher activity, selectivity and productivity useful for the oxidative coupling of methane to higher hydrocarbons.
Another objective of the present invention is to provide higher energy efficient and safe process for oxidative conversion of methane or natural gas to ethylene, ethane and higher hydrocarbons and for the oxidative dehydrogenation of ethane of C.sub.4 --C.sub.4 alkanes to ethylene and higher olefins employing the improved catalyst of this invention.